1. Field of the Invention
The present invention relates to a dispersion compensator of optical communications technology.
2. Description of the Related Art
Recently, a wavelength dispersion compensator, which compensates for wavelength dispersion in an optical communication by using an optical element called a VIPA element, has been studied and developed.
FIG. 1 shows the basic configuration of a wavelength dispersion compensator using a VIPA element.
The VIPA element is configured by forming reflection planes on both sides of a parallel flat plate of glass, etc., and by arranging an irradiation window for inputting light to part of a reflection plane. In a typical configuration, one of reflectances of the reflected planes is approximately 100%, whereas the other is equal to or lower than 100%, typically, 95% or so.
An optical signal used in an optical communication is input from a single-mode fiber to a line focuser. In the line focuser, the light output and spread from the single-mode fiber is once made into parallel light beams, and then collected into linear light with a cylindrical lens, etc. The light collected by the line focuser passes through the irradiation window of the VIPA element, and is collected on the inside of the VIPA element.
When the light that is collected into linear light is input to the VIPA element, light output from the opposite side has an output characteristic having angular dispersion like a transmission grating or a prism. Namely, light input to the VIPA element is reflected on the reflection planes of the VIPA element, and multiple-reflected within the VIPA element. At this time, since the reflectance of one of the reflection planes is lower than 100%, the multiple-reflected light is output from this reflection plane little by little. Then, output light beams interfere with one another, so that light beams whose proceeding directions differ depending on wavelengths are generated. Here, a light beam output at each reflection appears to be output from a different virtual image VI when viewed from the output side of the VIPA element.
This light beam is collected by a focusing lens, and reflected on a mirror. The light beam then passes through the focusing lens and the VIPA element, and is coupled by the single-mode fiber that forms the line focuser. Here, attention is paid to one light beam. When the light beam is reflected on the mirror, passes through the focusing lens, and again enters the VIPA element, a difference exists between the optical distances of virtual image VIs as is known from FIG. 1 if the virtual image VI from which the light beam is output is made to differ from the virtual image VI to which the light beam that again enters the VIPA element is input, depending on a wavelength of a light beam. Therefore, an optical distance traveled by a light beam differs depending on a wavelength. That is, since a distance traveled by a light beam differs depending on a wavelength, the light beam is output from the VIPA element with a propagation delay according to the wavelength. Accordingly, a light beam having a different wavelength undergoes different wavelength dispersion (chromatic dispersion), and is output from the VIPA element. A dispersion compensator implemented by using this phenomenon is a wavelength dispersion compensator using a VIPA element.
However, if a coupling loss of light to this wavelength dispersion compensator is minimized, a level of an optical signal differs depending on a wavelength and is output if such a wavelength dispersion compensator is used for an optical signal like wavelength-multiplexed light. This is because an insertion loss transmission characteristic of the VIPA element is not flat. Especially, in a wavelength-multiplexed light communications system that has been put into practical use in recent years, it is desirable that light having each wavelength maintains a predetermined optical level, and is transmitted. Therefore, optimization such as flattening of an insertion loss wavelength characteristic of a transmission channel band of an optical element, and by extension, flattening, etc. of an insertion loss of a transmission channel bandwidth of the periphery of the dispersion compensator, its system, and entire network must be performed.